Homelab

Experience. But it all depends. TDP gives a hint in some generations example epyc cpus TDP is relaivly acurate wheras the 35w tdp of intel second gen means litteraly anything between 10 and 60w.

But for consumer systems with an i or r5 cpu and a mid tier gpu 500w is enough. For a server without a gpu and similar cpu 300W will suffice.

dual socket systems of that cpu class maybe 400-500.

With more enterprise stuff 500-700 or with epyc maybe encroaching 1000w with some addin cards.

It depends on maany other variables like Drives attached, PCIe devices used etc.

I always add up all the power usage (ie cpu, video carde ect) then add 20-30%

A bigger one won't use any more power than a smaller one and are generally more efficient. Platinum is more efficient than Gold, which in turn is more efficient than Bronze.

Graphics cards really suck up power. Allow for the number of disk drives to have too.

Tdp seems fine to me for psu sizing. At the end of the day the psu may have some overhead anyway. You may want to look at pcpartpicker. They provide an indication of psu capacity

How close do you want to get? Budgeting about 200W per socket for “normal”-ish CPUs and 400-450W per socket for latest EPYC should get you in the right range.